L (to Mello): Kira is vengeance. Selfish vengeance. You're just two people who have thrown their toys out of their cot because they feel injured. Kira is angry at the world as he saw it and you're angry at him.
Forward has Inducer One-One-Nine, who is a type of psychic that is capable of Mind Manipulation. One-One-Nine is also a nine-year-old girl named Kathryn Wade, who was driven to insanity by her experiences at the Academy, to the point that in her fear of being recaptured she took control of a town and turned them into her personal killing horde, and later wiped out a space station with nearly a thousand people on board.
Harry in Heir— the severe abuse he receives from the Dursleys (along with the goading from the Tom Riddle horcrux in his head) pushes him over the edge. Also Tom Riddle / Voldemort is one because it's implied he had a psychotic break back when he was a boy attending an Orphanage of Fear when a girl posse killed his baby snake in front of himalaElfen Lied.
Harry again in Rise of the Wizards when he discovers that everyone he thought he could trust was lying and manipulating him. Over the course of the story he becomes darker and more paranoid as he is convinced that everyone is out to get him until he eventually seizes control of the wizarding world and becomes the next Voldemort in all but name.
In Mega Man: Defender of the Human Race, the female Robot Master in the beginning of episode 8 is activated by mistake, feels pain almost immediately upon waking, is unable to speak, overhears Wily talking about wiping her memory, and doesn't get a name before running off. She also has a mean shockwave that demolishes parts of Skull Fortress.
The story Faith and Doubt has the main villain Doubt as this. Doubt, at first glance, appears to be just another Enemy Within, whispering to Twilight and tricking her into being bad. It is only later that we learn that Doubt is a tragic figure who is the 6th Element of Harmony who lost his memory due to Celestia's actions. While he has the power to destroy the world (he can create alicorns on a whim) he gives the characters every chance to make things right and actually guides them towards stopping him. By the time the 19th chapter roles around and he is dying, all the characters have come full circle and now stand at his bedside, trying to save him. He gets better, reverting to his original form as Faith due to Twilight.
Rarity in The Secret Life of Rarity counts as this. At first she is merely a serial killer for the fun of it when she discovers that she gets a power fix when she murders, but eventually she turns into one when it begins to affect her friends and family in a negative way. By then she can't stop because she has become so addicted to the power rush she gets when killing. It gets to the point where she hates herself after killing but she has so little control of her urges that she has to keep going when she sees somebody hurt her friends and family.
Fluttershy, of all ponies. A Devil in Plain Sight and leader of the Religion of Evil, she also suffered from a veryAbusive Parent that left her with issues. (It involved being found by robot wolves as a child. Who were built by her father. So he could murder them while leaving a disturbing message about love. With that message causing her to be viciously attacked by insects.)
When Sweetie Belle gets upset at her sister, she threatens that if she can't be loved, she will make the world fear her and her (robotic) wrath instead. (Rarity mocks it, and Sweetie Belle storms off in a huff instead.)
Inner Demons: One could argue that following the awakening of her Queen persona and her subsequent Face-Heel Turn, Twilight Sparkle qualifies, from her own perspective at least — she's convinced that her beloved mentor deceived her (which actually is true), and that her friends and brother abandoned and betrayed her (which is actually sorta true), so she's lashing out in anger at the world around her. And even if she didn't qualify beforehand, she definitely does qualify after Trixie — the only pony left she felt she could trust — is accidentally killed by Apple Bloom; after that, what little sanity she had left goes completely out the window, her powers fully consume her and transform her into an Alicorn, and she decides that rather than being Equestria's ruler, she'll be its destroyer. Celestia actually lampshades the definition of this trope when telling Shining Armor that his sister can still be saved from herself:
"There is a difference between a monster and a destroyer. A monster has no heart, a destroyer may just be heartbroken."
The Pony POV Series has the second Big Bad, Princess Gaia/Nightmare Whisper who is actually Fluttershy's Superpowered Evil Side brought on by her "daughter" Fluttercruel (whom she's Sharing a Body with) accidentally performing theBreak the Cutie her "father" (Discord) couldn't do in a well intentioned attempt to teach her about nature's cruel side. As a result, Fluttershy becomes Nightmare Whisper in the same way that Luna became Nightmare Moon and tries to take over the world in an attempt to make it a utopia. She's ultimately saved by the others, thankfully.
Equestrias First Human: Hellfire had once been a proud soldier in Celestia's army, until a dragon burned his wife and child alive. Celestia dismissed his desire for vengeance. Hellfire faked his death and slaughtered the dragon. However, feeling dishonored by Celestia, he decided to murder her and use forbidden spells to take over Equestria, his actions quickly shredding all sympathy for him.
In Team 8, The possibility of Naruto becoming one of these is discussed. Itachi is attempting to break Kurenai's mind with his Dojutsu by making her live and relive the killing of herself and her team by a mob. She breaks it by forcing the illusion to faithfully recreate Naruto's likely reaction if he were to see Hinata killed: releasing the Kyuubi of his own will and letting it destroy the village.
Was this written before or after exactly that almost actually happened?
Rosario Vampire: Brightest Darkness Act VI has one in Talon Ryashen, the "Fairy Tale killer." He was just an ordinary boy until Fairy Tale kidnapped him off the streets and used him as a Super Soldier experiment, turning him into a Half-Human Hybrid by injecting him with Alucard's blood. He escaped the facility when the experiments were completed, and was rejected by his family because of what he had become; as a result, he has embarked on a crusade to kill anyone and everyone who is or was part of Fairy Tale, including Unwitting Pawns, receptionists, and those who turned on the organization, such as Dark, Akua, and Kahlua Shuzen. As he puts it, Fairy Tale took everything from him, and he has nothing left to live for but revenge on the organization.
The End of Ends tries to pull Beast Boy off as that, after he falls for the power of the Dark Prognosticus and tries to destroy the universe. His motivation for doing so is because he can't take life anymore after Terra does not want to come back to the team, following the ending of the show. Subverted heavily by the fact that he comes more off like a whinny little child who thinks breaking up with a girl is worse than losing your parents (or to compare to the Whole Plot Reference, losing your love) and wants to see everyone suffer.
Touhou Ibunshu has Remilia, Flandre, Mokou and Yukari as varying flavours of "death to everyone" due to a long life (or unlife, in the case of the former two) of despair.
Subverted in Imperfect Metamorphosis with Rin Satsuki, who despite suffering so much pain and abuse from nearly everyone around her since childhood she still doesn't want to hurt anyone, and her first action in the story is simply to discover why she had to suffer. It's everyone else who thinks she's this trope.
As the story goes on, she seesaws further into and out of the "Destroyer" bit, thanks in part to a) the aforementioned witch hunt, b) her isolation induced mental instability, and c) having absorbed a particularly sadistic Fallen Angel. After devouring Nue and Mamizou she very definitely qualifies for this trope.
Heavily implied with Meiko in Next On, but never actually stated-she says her sword drives people insane and that she stole it, and then she asks Miku to kill her.
New Dawn has a surprising one: Andras the Devourer. At first, he seems like an expy of Lavos, complete with a countdown to a catastrophic awakening event, and being the source of a type of magic. Specifically, a type of magic that relies on turning your pain into destructive power. It turns out, though, that Andras has a very, very good reason for wanting the universe gone. First, he was a human bully who did all he did because his father never acknowledged him, he was a product of an affair, and his mother was always absent and inferred to be "selling herself". He became "strong" in order to make sure he could never be hurt by his peers. And then he falls into the Magic Side. He is abducted by Queen Anathema, given a very sloppy brainwashing that results in painful clashes between his new and old identity, he is the most abused of her "knights", he was hated by all the other ones, and then he was thought dead when she dropped him into "The Devouring Pit", a pit full of demonic entities. What happened was much more cruel than if he was just killed. The creatures bound themselves to him and "he sank down into a lonely hell, a place where his only company was the echo of his screams". His own misery results in even more monstrous things merging into him in the course of his madness, resulting in the entity that would come to be known in the future as Andras the Devourer, or The Forsaken King. He is tortured, day by day, with even more Lovecraftian Superpower through the constant elevation of his power. By the time his arc comes around, he just wants it all to end.
This is one way fanfic writers go about having their favorite Axis Powers Hetalia characters perform actions analogous to wars, while keeping them sympathetic - pile on lots of trauma and then watch them snap.